Magnesium hydroxide slurries may be produced from magnesia (MgO) as a feed stock. Magnesia may be obtained from the natural mineral magnesite (MgCO.sub.3) or from sea water or brine. Production of magnesium hydroxide from magnesite generally involves calcination to decompose MgCO.sub.3 to MgO with subsequent hydration in water to convert MgO to Mg(OH).sub.2. The production of magnesium hydroxide from sea water or brine entails direct precipitation with quicklime (CaO) or more preferably with dolime or dolomitic lime. Commercially this is usually a precursor to the production of "synthetic magnesia" by subsequent calcination of the precipitated magnesium hydroxide. The sea water is seeded with magnesium hydroxide to promote crystal growth and to improve settling and filtering characteristics of the magnesium hydroxide precipitate. A flocculating agent is also usually added. The resulting thickened slurry is vacuum-filtered to produce a filter cake containing about 50% magnesium hydroxide. Because nearly all the magnesium hydroxide produced in this way is presently calcined to MgO, the characteristics of the magnesium hydroxide precipitate are optimised for solid/liquid separation rather than for magnesium hydroxide production.
Conventional magnesium hydroxide slurries may be utilised for desirable applications which include:
(a) use in relation to treatment of industrial effluent or waste water; PA1 (b) use as a commercial alkali for pH control; PA1 (c) flue gas desulphurisation; and PA1 (d) use of adsorptive properties of magnesium hydroxide to remove inorganic or organic contaminants. PA1 (a) JP 5-279017 and JP 5-279018 which describe a hydration process of light burned magnesia which is introduced into a hydration tank equipped with a stirrer or agitator and which is simultaneously milled by steel balls or other form of abrading apparatus; PA1 (b) JP-2-48414 which refers to a process of producing magnesium hydroxide as a hexagonal plate like crystal having a specific surface area (SSA) of 10m.sup.2 /g or less. A slurry of Mg(OH).sub.2 having a solids content of 5-70% wt % is formed as an intermediate in a hydration process at above 50.degree. C. under agitation and some slurry is periodically removed and replaced by hot water and magnesia to obtain a uniform slurry density. This slurry is used for producing high purity dry Mg(OH).sub.2 used for flame retardant applications; PA1 (c) JP-01-212214 refers to a method of manufacture of magnesium hydroxide wherein magnesia having a mean particle diameter of less than 100 micron is hydrated in the presence of alkali metal ions and/or alkaline earth metal ions and also in the presence of the hydroxide ion, nitrate ion, carbonate ion, chloride ion and/or sulphate ion. The magnesia is initially calcined between 400-850.degree. C. before being pulverised. A slurry is formed having 10-50% wt % Mg(OH).sub.2 ; PA1 (d) JP 3-252311 refers to a process for preparing "active" magnesium hydroxide wherein light burnt magnesia is obtained by calcining a natural magnesite at 850-1100.degree. C. and subsequently grinding the magnesia to a mean particle size of 5-10 micron and then subjecting the ultra-fine powder in an "acidic reaction" to obtain active magnesium hydroxide; PA1 (e) Bron et al., Chemical Abstracts (CA) 68(2): 5884e (1966) refers to the hydration of magnesite derived MgO. Magnesium hydroxide was produced during boiling or short wet grinding of the MgO with water in a ball mill and had a characteristic brucite structure consisting of hexagonal laminar crystals; PA1 (f) JP 54150395 which refers to production of magnesium hydroxide slurry by grinding dried magnesium hydroxide to a specified particle size and then mixing with water under agitation; PA1 (g) EP 0599085 which refers to an apparatus and method for the production of active magnesium hydroxide. In this specification coarse light burned magnesia is comminuted in the wet state with a wet-pulveriser and hydrated in the presence of an alkaline aqueous medium which included sodium hydroxide at an elevated temperature of not less than 70.degree. C. The resultant pulverised material is classified into fine and coarse particles using a classifying means which is generally set to restrict the passage of particles in excess of 20 micron. Subsequently the coarse particles are recycled to the wet-pulveriser. By subjecting light burned magnesia to concurrent wet-pulverisation and hydration in the presence of a heated alkaline aqueous medium, magnesia can be simultaneously comminuted and hydrated under rapid heating to produce an active magnesium hydroxide showing a low viscosity even at a high concentration; PA1 (h) KR 9301256 which describes formation of active magnesium hydrate made from light burned magnesite which is subjected to wet crushing with water, an alkali stabiliser inclusive of sodium hydroxide and dispersing agent inclusive of polycarboxylate using reaction heat and crushing heat; PA1 (i) DD 272288 which describes hydration of MgO resulting from MgCl.sub.2 thermal decomposition which is carried out by (a) pre-hydrating MgO in one or more series or parallel connected hydration reactors; and (b) grinding in one or more series or parallel connected hydration reactors; and PA1 (j) JP 03-60774 which refers to the production of magnesium hydroxide slurries which includes the step of slaking finely pulverised light burnt magnesia which is obtained by firing naturally produced magnesite with water with heating to 85-100.degree. C. Sodium hydroxide is added as a hydration accelerator. PA1 (A) high-solids-content, to allow high reaction capacity per unit (weight or volume) of slurry; PA1 (B) high reactivity; PA1 (C) stability to both settling and ageing; and PA1 (D) low viscosity to enable the slurry to be transported, sprayed, coated, (slip) cast and/or dosed by conventional pumping techniques. PA1 (i) having a solids content of between 40-80 wt % based on the weight of the slurry with the balance being water; PA1 (ii) containing between 0.01-5.0 wt % of at least one viscosity modifying agent or dispersant based on the weight of the slurry selected from the following groups: PA1 (iii) being sedimentation stable for at least 7 days without substantial agitation; and PA1 (iv) maintaining a viscosity of less than 1000 cP at a shear rate of 139 sec.sup.-1 over a period of at least seven days. PA1 (i) subjecting calcined magnesia of a particle size of about or less than 25 mm to a first particle reduction zone wherein said calcined magnesia is mixed with water and ground; PA1 (ii) passing said finely divided material through a screening means to ensure passage therethrough of finely divided magnesia having a particle size of about 2 mm or less and returning oversize magnesia particles to said first reduction zone; PA1 (iii) hydrating said finely divided magnesia in a hydration zone wherein said finely divided magnesia is mixed with water under agitation and heat so as to produce a magnesium hydroxide slurry having at least 80% hydration; PA1 (iv) passing said slurry though a second particle reduction zone so as to produce slurry particles wherein 90% of said slurry particles have a size of less than 50 microns; PA1 (v) adding a viscosity modifying agent selected from groups (1), (2), (3) and (4) prior to or during steps (i), (ii), (iii) or (iv) so as to ensure a maximum viscosity of 1000 cP; and PA1 (vi) obtaining after step (v) as a final product stable, pumpable, magnesium hydroxide slurries having a solids content of at least 40%. PA1 (i) continuously feeding magnesia to a hydration zone wherein the slurry is subjected to continual agitation; PA1 (ii) hydrating said magnesia in the hydration zone under agitation and heat so as to produce a magnesium hydroxide slurry having at least 80% hydration; PA1 (iii) adding a viscosity modifying agent selected from groups (1), (2), (3) and (4) prior to or during steps (i), (ii) or (iii) so as to ensure a maximum viscosity of 1000 cP; and PA1 (iv) obtaining as a final product stable, pumpable magnesium hydroxide slurries having a solids content of at least 40%. PA1 (1) inorganic acids having a molecular weight less than 130 amu and their inorganic salts excluding H.sub.2 SO.sub.4, H.sub.3 PO.sub.4, silicic acid and salts having an alkali metal as sole cation; PA1 (2) low molecular weight (i.e. less than 200 amu) carboxylic acids optionally containing one or more hydroxyl groups and their inorganic salts excluding salts having alkali metal as sole cation; PA1 (3) polyhydric alcohols or carbohydrates containing two or more hydroxyl groups and having a molecular weight less than 500 amu; and PA1 (4) alkaline earth oxides and/or hydroxides.
In this context magnesium hydroxide slurries are preferred for use in applications such as (a) (b) (c) and (d) above rather than magnesia either to avoid complications that uncontrolled hydration can cause, such as heat generation and cement formation, or because conversion of MgO to Mg(OH).sub.2 is essential to the desired application. In this regard magnesia in its reactive form in an aqueous environment will become hydrated to Mg(OH).sub.2 relatively spontaneously and in some cases it is desirable to control the rate of hydration dependent upon the desired application.
Magnesia in its reactive form commonly known as caustic magnesia, caustic calcined magnesite or light burned magnesia has to be distinguished from a relatively inert form of magnesia known as dead burned or inert magnesia.
None of the commercially produced magnesium hydroxide slurries which have a relatively high solids content (i.e. 50-60 wt %) have been found to have a combination of high solids content, stability to settling and high overall reactivity which are characteristics of magnesium hydroxide slurries produced by the present invention as hereinafter described. It is common practice in some conventional slurries, as referred to in prior art hereinafter, to incorporate a polymeric viscosity modifying agent to thin and/or stabilise the slurry but this practice has undesirable side effects as described hereinafter.
Reference also may be made to conventional processes for producing magnesium hydroxide slurries which are described in the following patent specifications:
It is known from JP 5-208810 and JP 3-252311, for example, that magnesia may be produced by calcination of magnesite followed by particle reduction to obtain ultra-fine particles having a mean particle size of 5-10 micron which is then hydrated to form a magnesium hydroxide slurry. The hydration process can be carried out in a particle reduction zone.
It is also known to add viscosity modifying agents or dispersants to the slurry to control viscosity or dispersability of the end product. Such viscosity modifying agents or dispersants can include. decomposable phosphates (see FR 2399485); carboxylic acid type polymeric surfactants (see JP 5-208810); polyanion and anion of strong acids such as HCl, H.sub.2 SO.sub.3 or H.sub.2 SO.sub.4 (see DE 4302539); polymeric anion dispersant and water soluble alkali metal salt (AU 48785/93); sulphomethylated acrylamide homopolymers or copolymers (see U.S. Pat. No. 4,743,396); alkaline salts of a sulfosuccinic ester product (see DE 3323730); alkali metal silicate and hydroxide and/or mineral acid salts (see J62007439); organic or inorganic dispersants (see J61270214); xanthan gum and lignin sulphonates (see CA 110 (10):7837e); carboxymethylcellulose (see CA 104 (6):39729k); cationic polymers (see U.S. Pat. No. 4,430,248); ferrous hydroxide or aluminium hydroxide (see CA 79(8):44013S) and polyacrylates (see U.S. Pat. No. 4,230,610).
It is also known to use additives to accelerate the hydration of MgO to Mg(OH).sub.2 and/or to modify the crystal shape of the magnesium hydroxide product during hydration. Such additives include citric acid or magnesium chloride (see CA 110(24):215623f), short chain carboxylic acids or corresponding salts such as magnesium acetate (see JP 3-197315, JP 01-131022 and DD 280745), ammonium chloride (see DD 241247); magnesium chloride, magnesium acetate, magnesium sulphate or magnesium nitrate (see DD 246971); inorganic or organic acids such as HCl or acetic acid or their magnesium salts such as magnesium chloride, or magnesium acetate (see CA111(18):159019n), proprionic acid (see JP 63-277510), n-butyric acid (see JP 63-277511), and sodium hydroxide (see JP 03-60774).
In particular, reference should be made to JP-3-197315, which refers to the production of a magnesium hydroxide slurry having 3-70 wt % and more preferably 20-50 wt % solids as an intermediate in the production of magnesium hydroxide crystals having hexagonal plate-like crystals which are obtained as a final product of the hydration of magnesia. These crystals are utilised as a fire retardant. JP-1-131022, which is discussed in the prior art preamble of JP-3-197315, states that the purpose of addition of magnesium salts such as magnesium acetate or organic acids such as acetic acid, is for controlling the rate of hydration or for controlling the growth of magnesium hydroxide crystals. The crystals that are obtained by the hydration process of this reference are regular in shape thereby avoiding the formation of agglomerates.
In general, desirable properties that magnesium hydroxide slurries should possess include:
The solids content of a magnesium hydroxide slurry is ultimately limited by its viscosity, which in turn is influenced by the particle size, shape, porosity, the number of particles, and the strength and nature of the particle-particle interactions and the particle-carrier interactions (in the present case, the carrier being water). In general, the greater the number of particles (and therefore interactions) and the greater the attractive nature of the interactions, the higher the viscosity and the lower the solids content achievable.
The reactivity is influenced by the surface area and particle size, with smaller particle sizes and higher surface areas favouring higher reactivity.
Stability to settling is mainly a function of particle size, with smaller particles giving greater suspension stability in the absence of viscosity modifiers.
Stability to ageing (i.e. no significant change in physical properties especially viscosity) appears to be related to the degree of hydration of the magnesia. It is known that substantially full hydration (i.e. greater than 98%) of MgO is difficult to achieve simply by suspending MgO in water. When MgO is hydrated in aqueous suspension, Mg(OH).sub.2 crystals tend to form on the surface of the MgO particles, thereby restricting access of water to the unhydrated MgO surface. As a consequence, the rate of hydration becomes progressively slower and a non-homogenous product can result. The properties of the product can vary with time, as the degree of hydration slowly increases. When the magnesium hydroxide is precipitated from sea-water, the product is, naturally, completely hydrated.
The aforementioned prior art refers to the use of additives including an alkali metal cation. It has now been established in experimental work referred to hereinafter that such additives, in particular sodium hydroxide, sodium acetate, sodium carbonate, sodium silicate and sodium chloride result in immediate thickening of untreated slurries or have no effect on the slurry and thus such additives are unsuitable for use as viscosity modifying agents.
It has also been established that the use of polymeric viscosity modifying agents, such as polyacrylates, have undesirable side effects in use. One side effect is that addition of a polyacrylate has the effect of first thickening the slurry to a paste and subsequently thinning the slurry following further addition of polyacrylate. The formation of a paste may cause a problem in an industrial slurry production process. Once a paste has been formed, it would be difficult to mix in further quantities of polyacrylate to fluidise the paste. Another side effect is that following addition of polyacrylate, the slurries were less reactive than the untreated slurries without polyacrylate. Another side effect is the propensity of some polymers to precipitate at low pH.